1. Field of the Invention
The invention relates to antenna arrangements for installation under dielectric covers, and more particularly to antennas for use in motor vehicle structures.
2. Description of the Related Art
It is common for motor vehicles such as cars, trucks, tractors, recreational vehicles and the like to use several antennas for such purposes as cellular telephones, CB, global positioning system (GPS), weatherband (WB), and the standard AM/FM radio. This proliferation of antennas is attended by special problems such as finding an appropriate mounting position for non-interfering operation as well as such inconveniences as high-speed antenna noise or “whistle.” Attempts have been made in the prior art to avoid external antennas and incorporate them into windowpanes and roof panels and the like.
Non-conducting materials such as fiberglass are now commonly used particularly in the construction of truck cabs in order to save weight. However, the use of such a dielectric material presents a problem for antenna designers, since most antennas require a ground plane provided by the metallic vehicle body for efficient operation. One solution to this problem is provided in U.S. Pat. No. 5,402,134 where a flat plate antenna module is disclosed. The module comprises one or more antenna loops formed from thin conductor strips arranged on a dielectric substrate. The substrate readily fits between the headliner of a truck cab and a nonconductive roof panel.
The solution in the '134 patent works well over a nonconductive surface, but there is a noticeable degrading of performance in applications were the antenna module is disposed over a conductive surface, such as in a tractor where the headliner is strengthened with metal. Also, the manufacture of antenna modules with conductive ink is expensive.
Another common solution, especially for vehicles using the common AM/FM and WB frequencies is providing three receiving wires of different lengths laid horizontally parallel or flared from a single attachment point. The AM receiving wire would typically be 105 inches long, the FM 30 inches, and the WB 16 inches. These lengths are normally effective to render the wires (at least for FM and WB) about a quarter wavelength at midband. But such an antenna arrangement proves to be highly directional and subject to other noise generating sources that may be nearby, including wiring harnesses, etc.